INDICATING COATING FOR LOCATING FATIGUE CRACKS

• US 3,803,485 A
• Filed: 09/25/1972
• Issued: 04/09/1974
• Est. Priority Date: 02/16/1970
• Status: Expired due to Term

• Current Assignee: Nelson A. Crites, Samuel P. Chambers
• Original Assignee: Nelson A. Crites, Samuel P. Chambers
• Inventors: Crites, Nelson A., Chambers, Samuel P.
• Primary Examiner(s): Krawczewicz, Stanley T.
• Application Number:
  US05/291,690
• Time in Patent Office: 561 Days
• Field of Search: 324/65R,91,104 73/88A,88R,91,104,141 200/61.08 250/71T
• US Class Current: 324/693
• CPC Class Codes: G01N 27/205 in insulating materials

Abstract

Crack propagation through a substantially electrically nonconductive coating bonded to an electrically conductive base is monitored or detected by providing a multitude of reservoirs filled with an electrically conductive liquid within the coating so that cracks propagating through the coating will penetrate one or more of the reservoirs and fill with the liquid through capillary attraction providing an electric current path through the coating thus changing the coating'"'"''"'"'s resistance to an electric current. By providing an electrical connection to the surface of the coating it is possible to detect the existence of cracking by noting changes in resistance or current flow between the metal base and the electrical connection. The invention is particularly useful for detecting cracks in the metal base which will propagate through the coating. A pigment or die to make the cracks visible may be substituted for or supplement the conductive liquid.

Claims

2. A method for determining the occurrence of crack propagation through a preselected area of a member between opposing surfaces thereof comprising:

a. providing a multitude of chambers within said member positioned between said surfaces so that propagating cracks will penetrate said chambers;

b. providing said chambers with a liquid having an electric current carrying capacity that exceeds that of said member so that cracks penetrating said chambers will be filled with said liquid;

making electrical contact with each of said opposing surfaces disposed so that said cracks will propagate between at least one pair of the electrical connections; and

d. monitoring changes in the electrical current carrying capacity of said member between opposing pairs of said contacts so that current paths caused by said liquid within propagating cracks will increase said current carrying capacity and indicate the presence of said cracks.

3. The method of claim 1 wherein said chambers consist of a multitude of capsules of an average diameter of less than 0.05 inch dispersed within the matrix of said member.

4. The method of claim 1 wherein said chambers consist of a multitude of capsules of an average diameter of less than 0.5 inch dispersed in a matrix juxtapositioned to said member.

5. The method of claim 1 wherein said chambers are positioned in one surface of said member.

6. The method of claim 3 wherein said chambers are affected by dispersing said capsules into the matrix of said member while said matrix is in a liquid state and solidifying said member from the liquid state to yield a solid member.

7. The method of claim 2 wherein said member is a coating that is applied to a metal substrate with said chambers being created while said member is in a liquid state and subsequently solidifying said coating, said metal substrate being one of said electrical connections.

8. The method of claim 7 wherein said metal substrate is a structural member that is subjected to cyclic stress and said coating consists of a material that has a greater fatigue resistance that said metal and elongation properties that do not exceed about 20 percent so that fatigue cracks occurring within said metal base or structural member will propagate through said coating to provide an electric current path.

9. The method of claim 8 wherein said coating consists of a cross-linked resin.

10. The method of claim 8 wherein the electrical connection opposing said metal consists of a coating having electrical current carrying properties.

11. The method of claim 8 wherein said chambers consist of capsules dispersed within said coating that have an average diameter of less than 0.050 inch.

12. The method of claim 8 wherein said chambers contain a pigment in addition to said current carrying liquid.

13. The method of claim 2 wherein said elecTrical connections and said member consist of coatings, said electrical connection being positioned on opposite surfaces of said member coating and all three of said coatings being sequentially applied to the surface of a relatively non-conductive base.

14. A structure disposed to indicate the existence of a crack comprising:

a. a multitude of chambers in proximity to a member with said chambers in a position where a crack in said member will penetrate said chambers;

b. liquid in said chambers capable of filling a crack in said chamber; and

c. means of detecting said liquid in said crack.

15. The apparatus of claim 14 wherein the means of detecting said crack comprises an apparatus monitoring the electrical conductivity between opposite surfaces of a matrix containing said chambers.

16. A structure disposed to indicate the existence of a crack therein comprising:

a. an electrically conductive base;

b. a substantially electrically nonconductive coating overlaying and bonded to an electrically conductive base, said coating consisting of a material having a greater fatigue resistance than the material of said electrically conductive base and elongation properties no greater than 20 percent;

c. a plurality of reservoir chambers positioned between the surfaces of said coating so that crack propagation through said coating will penetrate at least some of said chambers, said chambers being filled with a liquid having greater electric current carrying capacity than said coating;

d. electric current carrying means positioned on the surface of said coating so that cracks propagating through said coating from said base will fill with said liquid and form a current path between said conductive base and said current carrying means so as to increase electric current flow capacity between said conductive base and said current carrying means; and

e. means for measuring the potential electric current flow capacity between said conductive base and said current carrying means.

17. The apparatus of claim 16 wherein said chambers consist of microcapsules dispered throughout the matrix of said coating, said microcapsules having an average diameter of less than 0.05 inch.

18. The apparatus of claim 16 wherein said electric current carrying means include an electrically conductive coating overlaying said relatively electrically nonconductive coating.

19. The apparatus of claim 18 wherein said chambers are positioned between said electrically conductive coating and said substantially nonconductive coating.

20. The apparatus of claim 14 wherein said chambers are positioned in one surface of said member.

21. The apparatus of claim 14 wherein said chambers consist of a multitude of capsules dispersed in a matrix juxtaposed to said member.

22. A structure disposed to indicate the existence of a crack therein by use of a coating on said structure comprising:

a. an electrically conductive base;

b. a first substantially electrically nonconductive coating overlaying and bonded to said base;

said first coating having resistance to fatigue crack formation that is greater than said base and an elongation that is less than 20 percent;

c. a second coating overlaying said first substantially electrically nonconductive coating formed of a substantially non-conductive matrix containing a plurality of microcapsules, said microcapules being formed with electrically conductive walls and electrically conductive liquid cores, said capsules being placed in said matrix at a density to render said second coating electrically conductive, said matrix consisting of a material having a modulus of elasticity that is less than said base and elongation properties that do not exceed 20 percent, so that said cracks propagating through said first substantially nonconductive coating will propagate into said second overlaying coat to penetrate one or more of said capsules to fill said cracks with said conductive liquid and Provide an electric current path through said first substantially nonconductive coating changing the current carrying capacity through said first coating; and

d. means for measuring said change in current carrying capacity.

23. The structure of claim 22 wherein said electrically conductive liquid contains a pigment to render said cracks visible.

24. A structure disposed to indicate the existence of a crack therein comprising:

a. a metal base;

b. a coating overlaying said base capable of being cracked by the presence of a crack in said base with said coating consisting of a material that has a greater resistance to the formation of fatigue cracks than said base and elongation properties no greater than 20 percent; and

c. a plurality of reservoir chambers positioned between the surfaces of said coating so that crack propagation through said coating will penetrate at least some of said chambers, said chambers being filled with an indicating liquid disposed to render said crack detectable.

25. The structure of claim 24 wherein said indicating liquid is a dye disposed to fill a crack propagating through said coating and render it visible.